Particle accelerators, such as for example dielectric wall accelerators (DWA), are critically dependent upon high voltage, high current, fast closing switches that can be activated with nanosecond precision. Photoconductive switches offer the most potential in terms of handling such high voltages and high currents with minimum inductance, rapid closure, precise temporal control and the possibility of long life. Photoconductive switching is a technique where optical energy is applied to a semiconductor material, and the optical energy creates carriers within the semiconductor which renders it conductive.
The materials that have been used to date for photoconductive switch applications include Silicon and Gallium Arsenide (GaAs). The difficulty with these materials is that various failure mechanisms occur even at modest parameters. Further, the optical absorption depth for these materials is low, thus the carriers are forced to flow in a very thin area of the material bulk just below the surface. Thus, the principal issue with photoconductive switching has been short lifetime resulting from overstressing current and voltage conditions.
Additionally, silicon carbide (SiC) has long been a promising alternative candidate for use as a photoconductive switch material. It is only very recently, however, has this particular material been made commercially available in sizes and purity that merit application as a high voltage switch. SiC material has high dielectric breakdown strength greater than that of most solid materials (˜4 MV/cm), has high thermal conductivity (comparable to that of copper), and low optical absorption. Thus with the availability of single crystalline Silicon Carbide, a new class of switches is possible.
While promising, even SiC is subject to failure due to high electric fields locally produced where the electrodes separate from contact with the photoconductive substrate. FIG. 1 shows a photoconductive switch known in the art having a SiC photoconductive substrate 10 and two opposing electrodes 11 and 12. FIG. 2 shows an enlarged view of the box A in FIG. 1 showing the meniscus formed at the metal contact between the electrode and substrate surfaces. FIG. 3 shows the magnitude of the electric field on the contact surfaces, and illustrating the spike in magnitude at the triple points. Various methods have been employed to reduce and minimize these fields at such “triple points,” such as including filling the space between the electrode and substrate with a high permittivity material. This is shown in FIGS. 4-6. Similar to FIG. 1, FIG. 4 shows a SiC substrate 15 and two electrodes 16 and 17. Additionally, a high permittivity insulator 18 is filled into the space between the separation of the electrodes from the substrate. The meniscus 19 is shown in FIG. 5 at the triple point, where the triple point now includes the insulator material 18. FIG. 6, however, shows that there is still a spike, albeit with less magnitude, at the triple point of electrode-substrate separation.
What is needed therefore is a photoconductive switch for high voltage applications such as for particle accelerators, preferably implemented with a SiC material or other photoconductive materials, such as GaAs, that minimizes or at least reduces the high magnitude electric fields at the points of electrode-substrate separation.